(1) Field of the Invention
This invention relates to a process for preparing a thermosetting aqueous polymer emulsion.
(2) Description of the Prior Art
Increasing social demands for the prevention of environmental pollution and the saving of resources have tended to restrict the use of organic solvents in recent years. Therefore, the use of polymer emulsions using water as a medium has been noted in fields formerly using large amounts of organic solvents such as coatings, adhesives and so forth. Furthermore, the polymer emulsion using water has significant advantages in that it can be easily synthesized and, in addition, easily handled because its viscosity is much lower than that of the polymer solution using organic solvent having the same concentration. Therefore, polymer emulsions using water such as acrylic resin emulsion, polyvinyl acetate emulsion, ethylene-polyvinyl acetate copolymer emulsion and the like are used in a wide variety of applications, for example not only as a coating and an adhesive but also as a tackifier, finishing and treating agent for paper, fiber, woven fabric leather etc., and also as a bonding agent for non-woven materials.
However, the file obtained by drying the emulsion has an inherent fatal defect in that it easily absorbes moisture and swells and consequently its physical properties and adhesion to the substrate fall remarkably. And furthermore, as is clear from the film-forming process, the film is one in which the polymer particles are coalesced with each other, and it is, therefore, inferior to the solvent-cast film in the point of physical properties, especially, strength and toughness. Furthermore, the crosslinking of the conventional aqueous polymer emulsion has been effected by a condensation reaction of hydroxyl group in polymer particles several conditions are required in order to obtain crosslinking sufficient to provide a coating with satisfactory properties. Such conditions include lowering of the molecular weight of the polymer for effecting the permeation of the amino resin, the use of a large amount of the amino resin and baking for a long time at high temperatures. Therefore, the conventional aqueous polymer emulsion uses expensive raw materials and requires a long or complicated manufacturing process, baking process and so forth, it is not a satisfactory alternative to the conventional coating using organic solvents.
The emulsion polymerization of the radical polymerizable ethylenic unsaturated monomer by an irradiation with ionizing radiation is publicly known; however, when the system constituting a large amount of ethylenic unsaturated monomer containing high polar hydroxyl groups required for excellent properties of coating is used, it is very difficult to obtain a stable emulsion having polymer particles of the small diameter required for said emulsion in high yield, because the selective homopolymerization of the monomer in water is relatively high. Furthermore, it was not thought by anyone that crosslinking of polymer particles would occur if a system not including crosslinkable polymerizable monomer was subjected to emulsion polymerization by an irradiation with ionizing radiation, and that such crosslinking would improve significantly the coating properties.
A thermosetting polymer emulsion free from the various defects mentioned above has, therefore, long been desired by those skilled in the art.